In many applications of pulse generators, data generators, test instruments for digital circuits, etc., a manual or program-controlled adjustment of voltage levels of two logic states or pulse amplitudes is required.
In the prior art, the voltage levels have typically been controlled by a current distribution control circuit (see e.g., Hewlett-Packard Model 8160A Operating and Service Manual, page 8-152 and following). For this purpose, suitable analog multipliers have been used. The accuracy of the pulse amplitude adjustment with this type of circuit is dependent on the electrical symmetry of the semiconductor current switches over the range of the characteristics actually used. Also, the current divider branches must be symmetrical with respect to each other. In these prior art circuits, it has been difficult to keep the error below 1%, even if integrated circuits are used. Additionally, there is a tendency to overshoot and spurious peaks are coupled into the output signal.
It is also known to vary the current sum of a differential amplifier for generating the variable logic state levels according to the other control input of an analog multiplier circuit (see e.g., Hewlett-Packard Model 8160A Operating and Service Manual, page 4-57). A circuit operating according to this method is advantageous in that the current sum determining the amplitude can be precisely controlled. However, there are still problems with spurious peaks and overshoot.
Still another known circuit uses two variable voltage sources which are adjusted to the low and high logic state levels and are alternately connected to the output via an electronic switch, e.g., a field effect transistor (see B. E. Sear, "Computer Design," May 1972). A circuit of this type is advantageous in that it does not need a linear post-amplifier for providing the required output power of the pulse or data signal. In this case, the accuracy is limited by the bulk resistance of the electronic switches. In order to minimize this bulk resistance, field effect switching transistors having large area structures must be employed. Such switching transistors, however, cause coupling of non-negligible spurious peaks from the driving signal into the output signal via the transistors' internal capacitances.
In circuits for providing variable pulse rise times between given fixed logic state levels, it is known to employ a clamping circuit having diodes as switches. This circuit reasonably suppresses spurious peaks in the output signal, and overshoot in the output signal is also fully suppressed. However, if this principle is employed in a circuit for generating pulses with variable logic state levels, pulsating currents result in the two logic state level voltage sources. This can cause amplitude errors in the driving amplifiers for the two signal levels, particularly in the critical frequency ranges of the amplifiers.